1. Field of the Invention
This invention relates generally to an apparatus for preparing autologous vessels for use in bypass surgery and more specifically to a catheter and method for dilating the internal mammary artery or other arterial conduits for such use.
2. Discussion of the Prior Art
Internal mammary arteries are vessels which originate at the subclavian arteries and deliver blood to the sternum and musculature of the chest wall. These arteries along with gastroepiploic and inferior epigastric arteries, are being used increasingly in cardiovascular bypass surgery.
It is the purpose of a bypass operations to provide a passage from a site of free flowing blood to a site distal to an occlusion in a coronary artery. In the past, saphenous veins have been harvested from the legs of patients and grafted to the coronary arteries for this purpose. One end of the graft has been attached to the aorta which provides the source of the blood, while the other end of the graft has been attached to the coronary artery distal of the occlusion. In this manner, blood from the aorta has bypassed the occlusion to feed the heart muscle.
The internal mammary artery (IMA) is the most frequently used arterial conduit for bypass grafting. In harvesting the IMA from the chest wall, the endothoracic facia is sometimes incised from around the IMA to sever the constricting bands of circumferential fascia. While the IMA remains attached to the subclavian artery, the mobilized end of the IMA pedicle is attached to the coronary artery. An obvious advantage over use of the saphenous vein is the fact that only one end of the IMA pedicle needs to be reattached while the other end can remain in its natural position.
Unfortunately, the IMA has characteristics which cause it to spasm or constrict in response to the trauma associated with mobilization. It has been found that mechanical circumferential stretching of the IMA tends to overcome this spasm leaving a relaxed, larger diameter conduit for bypass surgery.
In the past the stretching of the IMA to overcome spasm has been accomplished by introducing a balloon catheter into the IMA and then drawing the catheter and the inflated balloon through the entire length of the artery. While this has tended to overcome the spasm, it has also resulted in denudation or stripping of the arterial intimal cell layer, sometimes referred to as the intima. This internal layer comprises important endothelial cells which line the interior of the IMA and allow the IMA to regulate its own diameter. For this reason, it is desirable to overcome the spasm without injuring or removing the intimal cell layer.
Procedures in the past have employed shear force gauges which measure the tensile force applied to the balloon catheter as the balloon is drawn through the IMA. While shear forces have been limited to approximately 30 grams, the intima is consistently damaged to an extent that has made this procedure clinically undesirable, and possibly damaging to the long term performance of the graft in the patient.